先序遍历
https://leetcode.cn/problems/binary-tree-preorder-traversal/description/

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void preorder(TreeNode *root, vector<int> &res)
    {
        if(root == nullptr)
        {
            return;
        }

        else
        {
            res.push_back(root->val);
            preorder(root->left, res);
            preorder(root->right, res);
        }
    }
    vector<int> preorderTraversal(TreeNode* root) 
    {
        vector<int> res;
        // 必须新起一个函数，否则每次都会重新定义vector
        preorder(root, res);

        return res;
    }
};

中序遍历
https://leetcode.cn/problems/binary-tree-inorder-traversal/description/

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void inorder(TreeNode *root, vector<int> &res)
    {
        if(root == nullptr)
        {
            return;
        }

        inorder(root->left, res);
        res.push_back(root->val);
        inorder(root->right, res);
    }
    vector<int> inorderTraversal(TreeNode* root) 
    {
        // 中序遍历默认树已经存在，不需要判空
        vector<int> res;
        inorder(root, res);

        return res;
    }
};

后序遍历
https://leetcode.cn/problems/binary-tree-postorder-traversal/description/

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void postorder(TreeNode *root, vector<int> &res)
    {
        if(root == nullptr)
        {
            return;
        }

        postorder(root->left, res);
        postorder(root->right, res);
        res.push_back(root->val);
    }
    vector<int> postorderTraversal(TreeNode* root) 
    {
        vector<int> res;
        postorder(root, res);

        return res;

    }
};